-LRB- CNN -RRB- -- The only sound Christian Myers heard in his classroom at Arapahoe High School was the constant buzz of vibrating cell phones -- calls from panicked parents trying to reach their children .

The sophomore and his fellow classmates were hunkered down in a corner of a darkened room .

Minutes earlier , they had heard the sound of gunshots .

That was followed by a dire warning from a teacher .

`` He just yelled , ` Lockdown ! ' '' Myers , a sophomore , told CNN on Friday . `` He goes ` Do what would you do in a lockdown drill . This is not a drill . '' '

` It was a shotgun '

It was just after 12:30 p.m. Friday at Arapahoe High School with its student population of more than 2,200 in Centennial , Colorado , when the shooting began .

Some students were on lunch break at the open campus ; others were still in classes .

Janitor Fabian Llerenas was on the north side of the building when he saw a student running `` side-to-side , kinda military style . ''

The student was carrying something . Llernas did a double take . `` It was a shotgun , '' he told CNN affiliate KUSA .

`` So right away , I got on the radio to alert everyone on the staff , '' he said . '' ... And that 's when I heard the shots . ''

' A really loud bang '

One minute , ninth-grader Whitney Riley was getting ready to retrieve her computer from her locker at Arapahoe High School . The next minute , she was running .

`` We were having fun and laughing and then , all of a sudden , we heard a really loud bang , '' the 15-year-old told CNN .

`` My teacher asked what it was and then we heard two more and we all just got up and screamed and ran into a sprinkler system room . ''

Inside the windowless room were five students and two teachers . `` We were shaking , we were crying , we were freaking out , '' Riley said . `` I had a girl biting my arm . ''

They soon heard people yelling , and walkie-talkies crackling , and then they heard police asking someone to drop the gun , put the gun down , and hold his arms up , she said .

She did not hear another gunshot , though the people doing the talking could have moved farther away , she said . Soon , they heard police ordering them out .

Three shots

Eric Waugh was in his physics class when he and his classmates heard a bang .

Initially , the teacher opened the door , he said . The teacher and students `` initially thought it might be an explosion from a chemistry class . ''

Then , they heard a gunshot . And then another . And then another .

The teacher locked the door and turned off the lights . `` He yelled at us to go and get in the corner , and we were in the corner in two seconds , '' Waugh said .

Later , SWAT officers came to the classroom door .

The students filed out in single file , their arms above their heads , he said .

`` We never thought this would happen at our school , '' Waugh said .

` Are you OK ? '

Myers , the sophomore , and his fellow classmates were sitting in the dark when he started to think about his twin brother , Austin .

He knew his brother should be in class . Even so , he was worried about him .

`` Are you OK ? '' Myers texted .

There was no answer .

He texted again : `` You doing OK ? ''

Still , there was no reply .

Myers was overwhelmed with fear . Was his brother one of those who had been shot ?

He tried several more times to raise his brother by text message to no avail . Then he sent a text message to a friend , who he knew had a class with his brother . Have you seen my brother ? Myers asked .

The answer : `` Austin never showed up to class today . ''

Panicked and in tears , Myers reached out to his father by text message : Dad , Austin is missing .

Moments later , his father responded : Austin stayed home today . He was n't feeling good this morning .

CNN 's Jake Tapper and Anderson Cooper contributed to this report .

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The shooting at Arapahoe High School in Centennial began just after 12:30 p.m.

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Janitor Fabian Llerenas saw a student armed with a shotgun , running military-style

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`` We were shaking . We were crying , '' 9th-grader Whitney Riley said

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Eric Waugh and his classmate thought initially it was an explosion in a chemistry class